By swapping out the sugar molecules that put the D in DNA, scientists have created new hereditary molecules that can undergo Darwinian evolution.

Making the six new molecules, collectively called XNAs, is a major technological advance that could lead to all sorts of new drugs, sensors and diagnostic devices. The research, reported in the April 20 Science, could also provide clues to how life evolved on Earth.

“What makes DNA and RNA so cool is they are the genetic molecules, they are the b...